The present invention relates to an antenna structure comprising a core member provided with an antenna element, and a cap fixed to the core member so as to cover the antenna element.
In an antenna for a mobile phone or the like, an antenna element is provided on an outer peripheral face of a core member in a substantially cylindrical shape which is formed of insulating resin, and a cap formed of insulating resin is fitted around this antenna element to attain protection of the antenna element. Then, the cap is fixed to the core member.
A first example of a related-art antenna structure for fixing the cap to the core member will be described referring to FIG. 12.
A core member 10 formed of insulating resin or the like has, at its base end side, a tongue piece 10a for restraining an escape of the core member 10 when it has been inserted into a bore 12a in a casing body 12, and at its distal end side, an outer peripheral face 10b in a substantially cylindrical shape. An antenna element 14 is arranged on this outer peripheral face 10b. In addition, a ridge 10c is provided on an intermediate part in the outer peripheral face of the core member 10 so as to extend in a circumferential direction thereof, close to a position where the core member 10 is projected from the casing body 12. Further, a cap 16 formed of insulating resin or the like is fitted around the antenna element 14. This cap 16 is provided with a groove 16a, on its inner peripheral face adjacent to an opening end, so as to extend in a circumferential direction thereof. The cap 16 is forcibly press-fitted over the core member 10, and the ridge 10c is engaged with the groove 16a, thereby to fix the cap 16 to the core member 10.
Referring to FIG. 13, a second example of the related-art antenna structure for fixing the cap to the core member will be described. In this figure, same or equivalent members to those in FIG. 12 will be denoted with same reference numbers, and repetitive explanation will be omitted.
In this example, a male thread 10d is engraved on the outer peripheral face of the core member 10 in an intermediate part thereof, close to the position where the core member is projected from the casing body 12. On the other hand, a female thread 16b which is adapted to be screwed with the male thread 10d is engraved on the inner peripheral face of the cap 16 adjacent to the opening end thereof. The cap 16 is fitted over the core member 10, and the male thread 10d is tightly screwed with the female thread 16b, thereby to fix the cap 16 to the core member 10.
In either case of the related-art examples as shown in FIGS. 12 and 13, fixing and retaining force for the cap 16 depends on mechanical strength in a radial direction of the opening end of the cap 16 where the groove 16a or the female thread 16b is provided. This means that reducing a thickness of the cap 16 at the opening end may result in decrease of the retaining force. Therefore, under the present circumstances, an outer diameter d1 of the cap 16 at the opening end side is necessarily required to be larger than an outer diameter d2 of the cap 16 in the intermediate part and at the distal end side.
Because the outer diameter d1 of the cap 16 at the opening end side becomes large in the related-art examples, there is such an inconvenience that a mounting area of the antenna to the casing body 12 becomes large accordingly, and freeness of designing an outer appearance of the cap 16 is restricted. Moreover, in the first related-art example shown in FIG. 12, elasticity is required for the cap 16 to be press-fitted, and so, material for the cap 16 is limited to an elastomer of insulating resin. In the second related-art example shown in FIG. 13, a fixed position of the cap 16 in the circumferential direction of the core member 10 may be indefinite. This would be a drawback especially in a case where a logotype mark is put on the cap 16.